Abstract

                                In old-time agriculture the only method of increasing soil fertility and crop yields was by stirring the soil with manual labour, to produce a pulverised condition of the soil. It was observed by experience that a similar increase in crop yield could be obtained by the application of cattle dung. Cattle manure was the earliest ingredient known to increase the productive capacity of soil. Subsequently other plant and animal materials were known to have a similar property. During the earlier period when the science of plant nutrition had not developed so much as now, the function of manures was considered to be merely supplying humus, which could be taken up by the plants directly for their growth. This humus theory of plant nutrition held the field for a long time and retarded the progress, in the fields of both plant physiology and agriculture. It was by the work of Liebig in Germany, his contemporaries Boussingault in France and Lawes in England, that the humus theory of plant nutrition was replaced by the "mineral" theory. These epoch-making discoveries laid the foundation of agricultural science in general and agricultural chemistry in particular. Liebig stated that plants could take up nutrients in mineral and soluble form. Insoluble substances should be converted into a soluble form to be assimilable by plants. An artificial manure known as Liebig's patent manure was prepared by him and put in the market. This manure failed, because it contained only alkalies (potassium) phosphates and sulphates There was no nitrogen in it, because Liebig considered that it was not needed, as he thought that plants could assimilate nitrogen in the form of ammonia from the atmosphere through the leaves. Lawes, on testing the recommendations of Liebig found them to be erroneous. The mineral theory was in turn criticised by others. There was nothing wrong with the theory, but there were two pitfalls: i. e., (i) omission of nitrogen and (ii) conversion of soluble potash and phosphorus compounds into an insoluble state by fusion with lime to prevent them from being leached out with drainage water.

Key words :